1. Field of the Invention
The present invention relates to switches for a load supplied by an AC voltage, and more specifically to a circuit for controlling an AC load using a low voltage control signal.
2. Description of Related Art
The control of an AC load from an AC voltage power system implies the use of a switch which is bidirectional both in current and voltage, that is, a four-quadrant switch. The present invention more specifically applies to such a switch made by means of two distinct switches for the two directions of the current, conversely to a triac, which is bidirectional. In an assembly with two one-way switches, the two switches may be openable (i.e., blockable) according to a constraint of opening under a non-zero current.
A problem which arises in any assembly using two semiconductor switches to make a four quadrant switch is the fact that one of the two switches must be controlled with a floating potential. This problem is illustrated by FIG. 1 which shows a conventional example of a four-quadrant switch 1 in series with a load 2 between two terminals E1 and E2 of an AC voltage source Vac (e.g., a main of 240V/50 Hz or 110V/60 Hz). In the example of FIG. 1, switch 1 is formed of two thyristors 3 and 4 mounted in antiparallel between a terminal A of load 2 and terminal E2, for example, the neutral of the AC power supply forming the assembly ground.
The use of thyristors implies an application in which it is not necessary to open switch 1 under a non-zero current, since thyristors are turned off by the disappearance of the current flowing therethrough. To have a possibility of opening under a non-zero current, blockable components, for example, MOS transistors, are used, as will be seen hereafter. Thyristors 3 and 4 are controlled from a control block 5 supplied, generally, by a low DC voltage Vdc provided by an auxiliary power supply. Control block 5 provides, in conventional assemblies, two control signals s1 and s2 intended, respectively, for thyristors 3 and 4.
The control of thyristor 3 by signal s1 raises no particular reference problem, since the anode of this thyristor is connected to the same ground as control block 5. However, the control of thyristor 4 must be performed via an isolation block 6, since the thyristor cathode is connected to node A. In the example of FIG. 1, isolating block 6 is an optocoupler, the emitting diode 7 of which receives, on its anode, signal s2 provided by control block 5 and the cathode of which is connected to the ground E2 common to control block 5 and to AC load 2 (via switch 1). Phototransistor 8 of optocoupler 6 is connected, by its collector, to gate g4 of thyristor 4 and, by its emitter, to node A.
The biasing of phototransistor 8 requires a DC auxiliary supply Vdc', different from supply Vdc of control block 5. Indeed, the emitter of phototransistor 8 must be connected to a reference potential M', which is necessarily different from the ground E2 of the rest of the assembly, and thus from the reference potential of voltage Vdc. Isolation block 6 thus is generally supplied by a voltage Vdc' referenced to potential M'. The operation of a bidirectional switch 1 such as shown in FIG. 1 is well known. Control block 5 organizes the respective conduction states of thyristors 3 and 4 according to the halfwave of AC supply Vac and according to a control reference which depends on the application.
A disadvantage of a circuit such as that shown in FIG. 1 is that the use of an optocoupler requires two galvanically isolated auxiliary power supplies Vdc and Vdc' having different references. To replace optocoupler 6 of FIG. 1, a pulse transformer 9 may also be used, as illustrated in FIG. 2. For simplification, FIG. 2 does not show all of the elements of the assembly of FIG. 1. Only transformer 9 is shown therein, the rest of the assembly is similar. A first winding 10 of transformer 9 is connected between terminal A and gate g4 of thyristor 4 (FIG. 1), that is, similarly to the connection of phototransistor 8. A second winding 11 of transformer 9 is connected between the assembly ground E2 and the terminal providing control signal s2.
A disadvantage of the solution partially illustrated in FIG. 2 is the use of a pulse transformer. Further, the pulse transformer used is a transformer operating at a high frequency as compared to the frequency of voltage Vac, to reduce the bulk. Control block 5 of the assembly must then provide high frequency control pulses. Further, a solution using a transformer such as illustrated by FIG. 2 excludes the use of transistors to form the switch. Indeed, to use transistors (for example, MOS power transistors) instead of the thyristors, the control signal cannot be an AC signal but must be a DC signal, which then requires rectifying the transformer signal and complicates the circuit. Another disadvantage common to the two conventional examples hereabove, be it an optocoupler or a pulse transformer assembly, is that neither of the two assemblies is integrable.
A control circuit for a bidirectional switch that has two unidirectional switches associated in antiparallel is disclosed in U.S. Pat. No. 5,796,599. More specifically, the disclosed control circuit controls the switches using two DC voltages that are generated at the terminals of two capacitors, each of which is connected to one of the switches. However, such a circuit requires two distinct DC voltages for controlling the thyristors of the unidirectional switches. Further, these voltages are referenced to different potentials that correspond to the respective power terminals of the switches. Thus, there is the absence of a common reference, and this makes it compulsory to include an insulating device (e.g., an optocoupler or transformer).